This invention relates to apparatus and methods for improving the in-spectrum dynamic range of tandem Time of Flight (“TOF”) mass spectrometers. Separation of ions prior to Time of Flight analysis has many existing applications.
According to a first example, ions may be separated by gas phase mobility (which in turn depends on shape and charge) allowing elucidation of structural information and/or removal of interference.
According to a second example, ions may be separated by mass to charge ratio (m/z) or mobility prior to fragmentation, reducing interference and improving confidence in assignment of fragment ions to precursor ions.
According to a third example, as packets of ions of equal energy produced by a travelling wave device travel into the pusher region of an orthogonal acceleration Time of Flight instrument, the constituent ions separate according to their mass to charge ratio. The timing of the Time of Flight pusher can be adjusted to allow enhancement in duty cycle optimised at chosen mass to charge ratios.
According to a fourth example, when the packets of ions described in the third example have been separated by ion mobility, it is possible to adjust the pusher synchronisation independently for each packet. Since mobility and mass to charge ratio are correlated, this results in an enhancement in duty cycle across the whole mass to charge ratio range.
The fourth example is an example of a High Duty Cycle (or “HDC”) mode of operation of an orthogonal acceleration Time of Flight instrument. For the purposes of the present application, HDC operation entails at least one stage of separation and packetisation according to a physicochemical property that is correlated with mass to charge ratio and synchronisation of the orthogonal acceleration Time of Flight pusher to optimise transmission of a particular mass to charge ratio value for each packet.
In many applications, ions are accumulated prior to separation to avoid loss of sensitivity. When the effects of ion accumulation, separation and improved duty cycle are combined for any particular species, the maximum ion current observed at the ion detector can be increased substantially. For low abundance components this results in improvement in the limit of detection, quantification and mass measurement. However, for high abundance species, the resulting ion current can exceed the dynamic range of the ion detector to the detriment of mass measurement and quantification.
Known methods of attenuation of ion signals typically reduce the transmission of all ions to some extent.
It is desired to provide an improved mass spectrometer and method of mass spectrometry.